Subterranean or other in-substrate acoustic sensors are known. Such devices have been used for metrology or the classification of the subterranean environment, for example, the identification of oil pocket location. Some such acoustic sensors have included piezoelectric instruments to detect subterranean sounds.
Conventional accelerometer sensors utilize a seismic mass that floats freely except for the attachment at a piezoelectric element. This configuration ensures that the device will be insensitive to vibrations, including those from sound waves, that strike the device off the primary axis, i.e., the line extending between the centers of gravity of the mass and the piezoelectric element. Accelerometers for metrology purposes are marketed by promoting this insensitivity to off-axis vibration, for example, that the device has a transverse (off-axis) sensitivity of 5% or less. It would be useful to have an acoustic sensor that could capture longitudinal as well as transverse vibrations for the acquisition of additional sound signal energy.